The present invention relates to inertial sensors, in particular accelerometers and gyros.
The invention relates more particularly to sensors comprising a sensing cell having a plate in which there are defined a portion forming a support member and at least one vibrating element associated with excitation means. The excitation means enable the vibrating elements to be set into vibration and enable its vibration frequency to be detected.
With an accelerometer, the inertial element is connected to the vibrating element. The inertial element is a mass, itself defined in the plate and movable relative to the support member. When the sensor is subjected to acceleration, the mass exerts a force on the vibrating element. This force modifies its frequency of vibration in such a manner that variation in the frequency of vibration of the vibrating element can be used for determining the acceleration to which the sensor is subjected.
Nevertheless, it has been found that temperature variations also give rise to variations in the frequency of vibration of the vibrating element.
In order to be able to obtain an accurate measurement of acceleration, it is therefore necessary to determine the influence both of acceleration and of temperature on the variation in the frequency of vibration.
To do this, it is known to associate a temperature sensor with the acceleration sensor. Nevertheless, because acceleration sensors of the above-specified type are compact, known temperature sensors need to be mounted separately from the sensing cell. Unfortunately, it is found firstly that the temperature gradient inside the sensor, and secondly that the time lag between a change in the temperature of the vibrating element and a corresponding change in the temperature of the probe are too great to enable the measured temperature to determine the influence of temperature on variation in the frequency of vibration in a manner that is reliable.
Document U.S. Pat. No. 5,020,370 discloses a transducer comprising a plate having two vibrating elements defined therein, with ends that are secured to support portions for fixing to a structure and to a mass. A temperature probe constituted by a conductive track extends over a beam connected to at least one of the support portions and extending parallel to the vibrating elements. Temperature is thus measured close to the vibrating elements. Nevertheless, the beam supporting the temperature probe runs the risk of affecting the mechanical behavior of the transducer, and thus of affecting the sensitivity of the transducer to acceleration.
An object of the invention is to provide a sensor that minimizes errors due to temperature variations.
In order to achieve this object, the invention provides a sensor for detecting acceleration, the sensor comprising a plate in which there are defined a support portion, at least one vibrating element carried by the support portion and sensitive to the movements to which the sensor is subjected, and at least one additional portion connected to the vibrating element and active in detecting acceleration, the sensor including a temperature probe comprising a conductive track which is fixed on at least the additional active portion and which is of resistance that varies as a function of ambient temperature.
Thus, temperature can be determined accurately and very close to the vibrating element, by measuring the resistance of the conductive track, and it can be taken into account in order to determine in reliable manner the respective contributions of temperature and of sensor movement in the variation of the frequency of vibration of the vibrating element.
The temperature probe preferably extends symmetrically relative to the vibrating element.
This minimizes errors that might result from a temperature gradient inside the sensor.
Other characteristics and advantages of the invention appear on reading the following description of a particular, non-limiting embodiment of the invention.